The trade winds in the Pacific have been
unusually strong since about 2000, a
study last year showed. And that’s
driving more
heat into the
deep ocean, rather than staying at the
surface.

So warming is continuing, but heat is being
rearranged within the vast oceans.

Pacific connections

Scientists think the shift in the trade winds is
being driven by a larger natural cycle, the Pacific Decadal
Oscillation (PDO) cycling between its warm and cold
phases.

A
new paper by Professor Kevin Trenberth from
the US National Center for Atmospheric Research and colleagues
explains:

“The Pacific Decadal
Oscillation was in a negative phase before 1976, but became
positive from 1976 to 1998, a period coinciding with strong
increases in global mean surface temperatures. Then it switched to
a negative phase in 1999, coinciding with the pause in the upward
trend in global mean surface temperatures.”

The positive (red) and negative (red) phases of
the Pacific Decadal Oscillation (PDO). You can see the shift from
positive to negative in 1999, leading to lower than average sea
surface temperatures in the North Pacific. Source: Trenberth et al.
(
2014)

Though this happens all the way out in the
Pacific, it has consequences across the world. The cooling pattern
disturbs atmospheric circulations locally, which then propagate in
a “wave-like” pattern from the Pacific right across polar regions
and the Atlantic, the paper explains.

The cooling effect in the Pacific is
particularly strong in winter, increasing the odds of cold winters
as the wave train makes its way across Europe, say the
authors.

What happens is the atmospheric waves emanating
from the tropical Pacific trigger a shift to the negative phase of
a second natural cycle, the North Atlantic Oscillation (NAO). The
paper says:

“In turn, this favours
cold outbreaks in Europe, as occurred in 2009-2010, 2010-2011 and
2012-2013.”

How about an Arctic link?

Scientists also have
a theory about what could be behind recent
cold winters in the UK.

Arctic sea ice is declining rapidly, with the
seasonal low in summer shrinking particularly quickly
. The loss of reflective sea ice is part of the reason Arctic
temperature has risen
three times faster than the global average
in recent decades, an effect known as Arctic
amplification.

As temperatures rise faster in the Arctic than
at lower latitudes, it’s thought this alters atmospheric
circulations, including the
jet stream – a stream of fast-flowing air in
the atmosphere.

The theory goes that as the jet stream slows
down and meanders more, Arctic air reaches further south and sticks
around for longer than usual, affecting the climate in places like
Britain.

This
Met Office video explains how the jet stream
affects the weather we see in the UK.

A scientific dispute

This is a very new area of research.
Another
new paper just published in Nature
Geoscience looks at the evidence for and against an Arctic
connection with Northern Hemisphere weather.

It suggests that while the mechanism still isn’t
clear, the evidence pointing towards a link is fairly compelling.
By the end of the 1990s, the frequency of unusually cold winter
months started to increase, coinciding with an acceleration in the
rate of Arctic warming. And the paper says:

“The seven years between
2007 and 2013 have exhibited the lowest minimum sea ice extents
recorded in September since satellite observations began … Several
of these seven winters following the low sea ice minima have been
unusually cold across the Northern Hemisphere”.

But there is still lots of uncertainty about
whether or not Arctic amplification can affect mid-latitude
weather so directly, the paper adds. Not least of which is a lack
of data in the Arctic.

Other studies have disputed a link too, says
the
Trenberth paper. The authors suggest an
alternative theory, that a warmer Arctic is a consequence of a wavy
atmosphere, not the cause.

The reason could be increased exchanges of cold
air from the Arctic with warmer air from lower latitudes, all a
consequence of the cooling in the tropical Pacific, the authors
explain.

A return to form

If the ‘hiatus’ in surface warming is largely
down to natural variability, we should expect a return to faster
warming at some point in the not too distant future. But while it’s
here to stay, the chances of a colder winter in the UK may have
just climbed a little higher.